Little is known about proteins which interact with membranes. The proposed research focuses on the structure and function relationships in a class of membrane-active, hemolytic toxins. We plan to model the structures of the thirteen known toxins in this family using computer graphics and potential energy minimization and then compare our predictions with spectroscopic data from methods sensitive to protein secondary structure (namely CD and Raman) and with the crystal structures we plan to determine. Small crystals of one of the toxins have been already obtained. The modelling will be based on the recently determined crystal structure of the protein crambin to which these toxins are homologous. The methods developed here can be used to predict the structures of other homologous series such as the immunoglobulins or the snake venom neurotoxins. By model building, we hope to distinguish between the structural and the functional homology of these proteins. In addition, we plan to chemically modify the toxins and test their toxicity so we may experimentally determine which features of these toxins are essential for toxicity and for membrane binding. We will test whether conformational changes occur when these proteins bind to lipids by using CD spectroscopy. Finally, we will label the toxins with fluorescent molecules so we can directly observe the binding of lipids to the toxins. This research should sharpen our tools for structure prediction, expand our knowledge of membrane-active proteins and provide us with a specific probe of membrane structure. These structures may help us understand how hydrophobic proteins such as the apo-protein from high density lipoproteins interact with lipids. Further, the membrane probe might be used to study platelet aggregation, since these toxins activate endogenous phopholipase A2 and effect prostoglandin synthesis.